In a typical isolated DC-to-DC converter, the control integrated circuit (IC) for regulation can be placed either on the primary side of the power transformer, where the input voltage is applied, or the secondary side of the transformer, where the output voltage is delivered to a load. When the controller is placed on the primary side, it is straightforward to obtain the required bias voltage and current from the input voltage during startup. This can be accomplished by a variety of means such as resistor dividers, simple linear regulators and so on. As long as the main power converter has been switched on, the required bias voltage and current can be derived from the main power transformer.
However, in general, it is preferred to utilize the secondary side alone for controlling the regulation of the output voltage since doing so normally produces the lowest distortion of the desired ramp-up curve together with efficient startup performance. When the controller is placed on the secondary side, during a startup phase of the converter, there is no bias voltage and current readily available from the transformer's secondary side to the controller and the associated circuitry. Thus, a smaller converter is placed on the primary side to power both the primary circuits and the secondary circuitry through an isolated output. This smaller converter requires a controller and a transformer to provide the isolated output for the secondary side. Unfortunately this extra converter adds additional cost for another transformer to meet the isolation requirements of the design and requires additional space on the printed circuit board, which causes critical concerns in terms of designing an isolated DC-to-DC converter.